Xeric element and method of preparing the same

ABSTRACT

The invention relates to a xeric element comprising the spine of the throwing arm of the seed pods of plants of the genus Geranium, and the method of preparing it for use in instruments which measure or respond to relative humidity or dryness. The plants of this genus literally &#39;&#39;&#39;&#39;throw&#39;&#39;&#39;&#39; their seeds and the spines of the seed pod are characterized by having a substantially uniplanar movement, a rapid response to changes in relative humidity, inherent strength sufficient to avoid breakage when bridled, and its warp movement is reversible on a substantially identical hysteresis curve on both adsorption and desorption. Preferably, the seed pods are gathered after the seeds are thrown, the throwing arms removed from the balance of the pod, the arms retted by soaking in warm water, and then the woody fiber coating and beards on the ribs of the arms are removed. Also preferably, such ribs are attached at their ends only to a cradle formed of a thin strip of beryllium copper.

United States Patent Fenner et al.

[451 Sept. 5, 1972 [54] XERIC ELEMENT AND METHOD OF PREPARING THE SAME[22] Filed: June 8, 1970 [2]] Appl. No.: 44,626

Related US. Application Data [63] Continuation-in-part of Ser. No.834,087, June 17, 1969, abandoned.

[52] US. Cl ..73/337 [51] Int. Cl ..G0ln 19/10 [58] Field of Search..73/337, 337.5, 336.5, 335, 73/29 [56] References Cited UNITED STATESPATENTS 3,135,117 6/1964 Fenner ..73/337 3,198,011 8/1965 Fenner..73/337.5 2,699,062 1/1955 Tomaschek ..73/337.5

OTHER PUBLICATIONS Gray s Manual of Botany rewritten by Merrit LyndonFerwald; 8th edition copyright 1950; pages 946 and 947 Just Weeds" byEdwin Rollin Spencer; page 141; copyright 1940 and 1957 PrimaryExaminerLouis R. Prince Assistant ExaminerDenis E. Corr Attorney-RobynWilcox [57 ABSTRACT The invention relates to a xeric element comprisingthe spine of the throwing arm of the seed pods of plants of the genusGeranium, and the method of preparing it for use in instruments whichmeasure or respond to relative humidity or dryness. The plants of thisgenus literally throw their seeds and the spines of the seed pod arecharacterized by having a substantially uniplanar movement, a rapidresponse to changes in relative humidity, inherent strength sufficientto avoid breakage when bridled, and its warp movement is reversible on asubstantially identical hysteresis curve on both adsorption anddesorption. Preferably, the seed pods are gathered after the seeds arethrown, the throwing arms removed from the balance of the pod, the armsretted by soaking in warm water, and then the woody fiber coating andbeards on the ribs of the arms are removed. Also preferably, such ribsare attached at their ends only to a cradle formed of a thin strip ofberyllium copper.

14 Claims, 8 Drawing Figures PATENTED 5m? 7 3,688,579

sum 1 BF 2 INVENTOR.

RALPH L. F was BY ATTORNEY TENTH 1 5 3.688.579

INVENTOR.

RALPH )%:ENNER BY 1 "g 2 ATTORNEY XERIC ELEMENT AND METHOD OF PREPARINGTHE SAME This application is a continuation-in-part of our pendingapplication of the same title, Ser. No. 834,087, filed June 17, 1969,now abandoned.

BACKGROUND OF THE INVENTION It has long been known that the seeds of thegenus Erodium, one branch of the Geranium family, have pronounced xericqualities and hence the seeds of that genus have been used to operateweather indicators, and more recently have been used as the operativeelements of xeric instruments. In contrast to the prior art, which usedthe corkscrew-like tail of the seed for a moisture-sensitive element,this invention relates to the seed-throwing extension, or arm, of theseed pod of the plants of the genus Geranium, which have better xericqualities than the seeds heretofore used. The seedthrowing arms of theseplants have superior xeric qualities in that they are much more rapid inthe speed of their response to changes in relative humidity of theenvironment; the hysteresis curves (curve of movement in adsorption ordesorption) is substantially the same, as compared to considerabledifferences in the older art; and the elements lie in the form of auni-planar spiral rather than the helix form of the seeds of Erodiumplants. When dry, these throwing arms of the pods of plants in the genusGeranium assume a shape such as that shown in FIG. 1 and form auni-planar spiral of between 450 and 540, and become straight (but notlimp) when saturated. The throwing arm of the seed pod can be used forsome purposes in its natural condition, but in its natural state is notas accurate as when the spine, or rib, of the arm is cleaned of theother material composing the arm, as the spine has a much greateraccuracy and much longer life, and is less readily affected by heat andpressure than the original arm.

OBJECTS It is an object of the present invention to provide an improvedxeric element.

It is another object of the invention to provide a xeric element whichhas a warp force in a single plane, i.e., forms a spiral, in contrast tothe helical members heretofore known.

Another important object of the invention is to provide a fibrous xericelement which has a warp force varying inversely with humidity withoutlongitudinal expansion.

It is still another object of the invention of to provide a xericelement which does not require the complicated modifying devices of theprior art when the element is used in an instrument.

A further object of the invention is to provide a xeric element whichdoes not acquire a set from prolonged exposure at moisture extremesoften found in measuring and scientific devices.

It is still a further object of the invention to provide a xeric elementwhich has minimum hysteresis on both adsorption and desorption cycles,has long life, a rapid speed of response and is small in size.

It is a further object of this invention to provide a xeric element ofimproved viscoelastic qualities.

It is another object of the invention to provide a xeric element withbetter thermal characteristics than heretofore known, so that warping isdue to moisture only and not a combination of temperature and moisture.

It is still a further object of the invention to provide a xeric elementwhich is readily incorporated in instruments for various purposes, suchas measuring relative humidity, controlling relative humidity, and thelike.

Another important object of this invention is. to provide a partiallybridled xeric element in which the xeric element is attached to a thinand resilient piece of metal at the ends only, whereby the warp force ofthe xeric element will not be adversely affected by thermal expansion orcontractions of the metal.

These and further objects of the invention will be apparent from thefollowing detailed description of the xeric element, taken inconjunction with the accompanying drawings in which:

FIG. I is a greatly enlarged plan view of the seedthrowing arm of theextension of the seed pod or husk of Geranium dissectum.

FIG. 2 is the arm of FIG. 1, shown in a fully saturated condition.

FIG. 3 is a view of the spine of the arm shown in FIGS. 1 and 2 afterretting and stripping and in the" dry condition.

FIG. 4 is the retted, and stripped, member of FIG. 3 shown in itsstraight condition, which is assumes when it is thoroughly saturated.

FIG. 5 is a cross-sectional view of the arm of FIGS. 1 and 2, such astaken along the plane indicated by the line 55 of FIG. 2.

FIG. 6 is a greatly enlarged perspective view of a copper cradle, orstrip, inwhich one or more of the vegetable xeric elements are inserted.

FIG. 7 is a perspective view of the completed device, or chip, in whichthree of the vegetable elements, while completely saturated, have beensoldered or otherwise retained.

FIG. 8 shows the chip of FIG. 7 in the position which it assumes whenthe vegetable elements have been permitted to dry.

The invention relates to a xeric element and the method of preparing itfor use in instruments which measure or respond to relative humidity, ordryness. The word xeric is used in its scientific and technical sense asindicating a condition of dryness, and is, therefore, commonly used torefer to devices or elements which are responsive to, or measure,relative humidity.

The device of the present invention is designed to provide a verysensitive and fast acting element suitable for use in measuring therelative humidity of the environment in which it is located. Such adevice can be used simply to measure the relative humidity of theenvironment, or it can be used as the sensing and actuating element of axeritron (a device for modifying resistance in an electric circuit andthus valuable in telemetering a registration of relative humidity, oroperating a signal in the event the relative humidity goes beyond somepredetermined point), or other devices in which a measurement ofrelative humidity is important.

I-Ieretofore it has been impossible to simply, and at the same time,accurately, measure or control moisture, or more specifically, therelative humidity of a particular environment. Even the use of the seedof the genus Erodium, which is described in Fenners patents, U.S. Pat.Nos. 3,135,117 and 3,198,011, required very complicated devices as shownin the latter patent. In the past, accurate methods of measurementrequired sensors combined with other instruments, such as the dry andwet bulb thermometers used by meteorologists in their determination ofhumidity in the atmosphere, and the readings thus secured were computedby the use of tables, complicated computations, and the like. Incontrast, the present invention utilizes the seed pods (not the seeds)of a genus quite unlike the common garden Geranium (which is one of thegenus Pelargonium) or the genus Erodium commonly used in the prior art.Specifically, the present invention utilizes the spine of the throwingarm of the seed pods of the genus Geranium (which does not include theplants commonly called geraniums", for they are members of the genusPelargonium, whose seeds and seed pods are similar to those of the genusErodium), and particularly the species Geranium dissectum, GeraniumRichardsonii, Geranium attenuilobum, Geranium maculatum, and Geraniumeriostemon. These seed-throwing arms of the seed pod, especially whenprepared as suggested ,below and as will be more fully describedhereafter, provide a simple and accurate measurement of relativehumidity which is accurate both in the adsorption and desorption cycles,which can readily be read, and which is completely reversible in itsmovement. The particular species mentioned above, and others of thegenus, have been tested and all are found to have sufficient strength,satisfactory life, reversible movement as the humidity of theenvironment changes, rapid response to change of humidity, and goodviscoelastic and thermal characteristics. However, it can be mentionedthat to the present time the pods of the Geranium dissectum have beenmost often used as that plant is most readily available in the SanFrancisco Bay area. However, the seed pods of the other speciesmentioned are preferred as they form the uni-planar spiral shown in thefigures, and the arms are much longer and stronger. Other plants of thegenus have been found to have the same characteristics, but in varyingdegrees. These elements respond substantially only to water and can beused in gas or liquid systems since they do not respond appreciably togas or larger molecules of the nature of alcohol or acetone, oils, etc.These elements likewise do not respond to changes in temperature or tochanges in gas pressure, except as such conditions influence water vaporpressure and have been found suitable for use in systems up to 3,000pounds per square inch.

It has long been known that the seeds of the genus Erodium, one branchof the Geranium family, have pronounced xeric qualities and hence theseeds of that genus have been used to operate weather indicators, andmore recently have been used as the operative elements of xericinstruments. For example, see Fenners prior patents above mentioned, andthe references there cited. Seeds of the genus Pelargonium (which,incidentally, includes the plants commonly called Geraniums), have thesame quality to a limited extent. This invention relates to theseed-throwing extension, or arm, of the seed pod of the plants of thegenus Geranium which have better xeric qualities than the seedsheretofore used. The plants of the genus Geranium literally throw theirseeds whereas the plants of the genus Erodium merely drop their seedswhen the seed pods mature, The best examples of such seed-throwingplants are those specifically mentioned above as well as Geraniumcaespitosum (sometimes identified as Geranium Californicum), Geraniumerianthum, and Geranium strigosus. The seed-throwing arms of theseplants have superior xeric qualities in that they are much more rapid inthe speed of their response to changes in relative humidity of theenvironment; that the hysteresis curve (curve of movement in adsorptionor desorption) is substantially the same, as compared to considerabledifferences in the older art; and the elements used lie in the form of auniplanar spiral rather than the helix form of the seeds of the variousspecies of Erodium and closely allied types.

The seed pods of the plants specifically mentioned generally containfive seeds circumferentially arranged around the base of the pod and liewithin divergent arms, or fingers, of an extension of the seed pod. Thefive arms grow together as a single conical extension of the seed pod,extending axially from the base which contains the seeds. When the seedsare fully matured, the pod dries; and when the pod has dried to thecondition nature desires for seed scattering, the warp force of theseextensions rips the pod open and each of the five extensions snaps totheir dry condition and literally throw the seeds in divergentdirections. When dry, these fingers, or what might better be called thethrowing arms of the pods, assume a shape such as that shown in FIG. 1and form a uni-planar spiral of between 450 and 540, depending to aconsiderable extent on the dryness of the environment. Since the seedpod itself bridles the warping force, that is, limits its movement untilthe warp force (the pWl) of the throwing arm exceeds the bridling forceof the pod, the arms literally snap to their spiral form and the seedsare thrown 15 or 20 feet when the warp force of the five arms exceedsthe bridling force of the pod. These throwing arms of the seed pod arethe xeric element of this invention. As indicated above, this throwingarm of the seed pod can be used for some purposes in its naturalcondition, since it, the seed-holding extension of the seed pod, assumesthe spiral shape when thoroughly dry and becomes straight (but not limp)when saturated.

However, as also indicated above, the element in its natural state isnot as accurate as when the rib, or spine, thereof is cleaned of theother material composing the arm, as the spine has a longer life, isless readily affected by heat and pressure, and has much greateraccuracy, since the coating of the spine will gradually deteriorate andthus gradually change its response to xeric conditions. The preferredmethod of cleaning such spines of the throwing arm is by retting thepods subsequent to their natural scattering of the seeds by placing themin a water bath having a temperature preferably of the order of about toF. The seeds are allowed to ret, or soak, in this water for a period ofapproximately 3 weeks to permit bacteria to attack the woody fibercoating the beards on the arm in its natural state. After such retting,the woody fiber coating and beards on the arm are readily removed byslight pressure, as by pulling the am between the fleshy parts of thefingers of an operator. After this stripping action, the rib, or spine,which is left is relatively straight because of its saturated condition,and the element is ready for incorporation in the desired instrument.

Preferably, the spine, or a plurality of them, are soldered into acradle formed by a thin strip of beryllium copper. Preferably, thecopper strip has a thickness of one one-thousandths of an inch, a widthof 2 millimeters, and a length of 14 millimeters each end of the stripbeing folded back for a distance of 2 millimeters to form a chipmillimeters (l centimeter) in length (FIG. 6). If two or three suchspines, or ribs, in a thoroughly saturated and straight condition, areplaced in the cradle, and each end dip soldered (FIG. 7), the elementsare rigidly secured at their ends to the copper chip, and due to theirsaturated condition, the solder has not harmed them in any way. Whendry, the chip will assume the form shown in FIG. 8 in which the naturalwarping force has been bridled, or limited, to an angle of 60.

The actual xeric element, i.e., the spine of the throwing arms of theseed pod of the genus Geranium, contains moisture, as do all organicfibers. Water in this fibrous arm is not all free water, but may existin either the bound or partially bound state. Properties of bound waterare considerably different from those of free water. Bound water vaporpressure is lower, and boiling temperature is higher; its freezingtemperature is lower, and its density is greater than that of freewater. Because the vapor pressure of bound water is less than that offree water, a dry xeric element will take up water until the vaporpressure of the absorbed water equals the pressure of the surroundingenvironment. As more water is taken up by the xeric element, the bondingenergies become less. In a space saturated with water vapor, theyultimately become zero and vapor pressure of the adsorbed water equalsthe saturation pressure of water at the same temperature. Moisture inexcess of this quantity is thermo-dynamically free water.

Warp in any xeric element is a function of bonding energies. At fibersaturation, the warp force exerted by the xeric element is zero, i.e.,it is balanced by vapor pressure. As the vapor pressure of thesurrounding environment is decreased, or the temperature increased, thebonding energy is increased and the xeric element generates a cellularstress proportional to the bonding stress. If the xeric element isunrestrained (commonly referred as being unbridled), the result in thethrowing arms of the plants of the genus Geranium is a radialdisplacement in the form of a spiral. This action is most pronounced inthe particular species above referred to. The warp force will reachequilibrium with the surrounding environment as the vapor pressure ofthe environment equals the vapor pressure of adsorbed water in the xericelement. In a completely dry environment at any given temperature andpressure, a state of equilibrium will be reached with the bound water.If the environment is maintained dry, and the pressure is reduced tozero, i.e., a vacuum, there will be a specific warp force index for anytemperature. The values obtained by this procedure represent the upperlimit of the warp force index (pWl). Thus, minimum and maximum warpforce are defined by water immersion and the specific pWI (warp forceindex) for a given temperature. Warp displacement, i.e., the angle ofdisplacement associated with a 1 percent change in relative vaporpressure, is greater at the high vapor pressure (high humidity) end ofthe displacement. It is almost linear between 30 percent and percentrelative humidity (R.l-I.). Below 30 percent R.I-I. the pWI is less thanR.H.', and above 80 percent R.l-I. the pWI is greater than R.l-I.

If the xeric element is completely bridled, i.e., restrained throughoutits entire length, the warp force is manifested as a dynamic tensioninstead of a radial displacement. The dynamic tension as expressed byresponse of a silicon strain gauge, for example, is much more rapid thanthat of the same xeric element in the free-turning state. A significantbehavioral aspect for measurement and control purposes is that a fullybridled element has a warp force that is linear over. almost its entirerange, specifically above 5 percent relative humidity and below 98percent. However, a fully bridled element is more sensitive totemperature changes which will affect the metal to which the element isbonded because of the disparity in thermal coefficients of expansionbetween organic cellulose and inorganic metals. When the xeric elementis partially bridled, i.e., attached to a resilient metal strip at itsends only, and in cups which permit linear compensation for differentialthermal expansion, the warp force is as good as that of a completelybridled element, yet the xeric fiber (i.e., warp force) is not affectedby thermal expansion or contraction of the metal, and throughout most ofits length the fiber is free from cement or solder which would interferewith its reaction to the water content of its environment. It is forthis reason that we prefer to use the xeric chips shown in FIGS. 7 and8.

If the xeric element of the present invention is not abused, it has anindefinite life and will retain its warp force response indefinitely. Ifthe element is subjected to a temperature in excess of C. (approximately250 F.), the pWI effect becomes irreversible, since water molecules arebeing driven off which were incorporated in the cellular structure byliving processes. Warp displacement increases to the point at whichincineration of the fiber occurs but it is irreversible after 125 C.

As indicated above, the xeric element of the present invention is whatmight be called the throwing arm 10 of the seed pod of the plants of thegenus Geranium, and particularly of the species Geranium maculatum,Geranium Richardsonii, Geranium attenuilobum, Geranium dissectum,Geranium caespitosum (sometimes identified as G. Californicum), Geraniumstrigosius (sometimes identified as G. nervosum"), Geranium erianthum,and Geranium eriostemon. The gynoecium of this genus generally containfive seeds which, during the development to maturity, are held in anindividual pocket 11 formed between two diverging fingers 14 at thelower. end of the throwing arm, or seed-throwing member 10. The fiverespective throwing arms are attached to a central stylar column at thebasal section directly above the ovule. The seed coat is connected tothe stylar column with radial cellular structure. This structure nails"the base of each individual throwing arm to the stylar column. The uppertips of the throwing arms, or xeric spines, are fastened solidlytogether and to the stylar column in the area directly below thestylodia, or free distal ends of the stylar column. Between these twopoints of attachment a longitudinal cellular structure analagous tosplitting alignment exists between the xeric element segment of the seedcoat and the stylar column. This fracture zone exists as a boundarylayer between the dorsal aspect of the xeric element segment of themature seed coat developed in the Geranium gynoecium and the stylarcolumn. The cellular structure in this boundary layer does not possessxeric properties, i.e., it does not warp. Thus, all five throwing armsare permanently attached to the tip of the central stylar column and areseparably attached at their dorsal sides to that column. Thesignificance of this mechanism is realized in the operation of thethrowing arm on maturity of the fruit, i.e., at seed maturity. First theseed coat containing the mature seed is ruptured when the wrist areadirectly above the pocket holding the seed contracts and pulls the seedfrom its position in the receptacle. This action opens the pod so thatthe seed lies loosely in a capsule with the bottom open. Continueddrying of the xeric element finally increases stress on the wristsection which is nailed to the stylar column and on reaching criticalstress level, the wrist is released. The released stress results incleaving the boundary layer between the xeric element and the stylarcolumn as the xeric element curls. This action results in an underhandthrowing action which casts the seed many yards. The resulting throwingaction is the method used by plants of the Geranium genus for seeddispersal. Such a throwing action is the result of the warp forceactuated by a molecular stress in the cellulose crystallite of thethrowing arm. It is this throwing arm, warping in response to conditionsof moisture and dryness, which we employ as the xeric element for thesensing of moisture.

Since the mechanism is in nature quite complex, the following analogy isused to explain the mechanism of the throwing arm: Visualize a baseballheld in ones hand, and the hand and arm extending straight down alongthe leg; the ball held toward the leg. The wrist of the hand is strappedto the leg with a breakable elastic band. First the wrist is turnedupward so the ball is held in a cupped hand, palm downward. Thenincreasing force is exerted against the elastic band until it breaks.

In its natural condition, each of the arms 10 is a vegetable elementwhich naturally is extremely xeric in nature and very resistant tooutside conditions other than moisture, such as gases, and nonhydrousliquids, such as oil, alcohol, and the like. This element is covered bya thin vegetable coating from which extend a great plurality of hairs,or barbs, which coating is subject to deterioration and preferably isremoved by the retting process to be described shortly. As mentionedabove, a warp force begins to develop in the throwing arms as the seedmatures. Since they are attached to the stylar column, motion ormovement of the arm is completely bridled until that time when thecombined force of the arm overcomes the release force at the base, orwrist. When the warp force exceeds that of the tension at the wrist, theboundary layer between a xeric element and the stylar column splits asif it had been struck with an axe and the arms 10 snap to theirnaturally dry position, which is shown in FIG. 1. This movement of thearms is so rapid and so strong that seeds are thrown up to distances offrom to feet.

This action can perhaps best be compared to snapping a pea or bean by afinger held in restraint by the thumb. After the seeds are thrown, theresidual gynoecium consists of the receptacle (not shown) where theseeds matured, a central stylar column (likewise not shown) extendingupwardly therefrom, and at the top of this column the several throwingarms 10 attached by their upper thread-like tips 12.

The seeds (not shown) develop in a small membrane, or pocket, 11 whichis cradled between the divergently extending fingers 14 attached to, andintegral with, the lower end of the arm 10. When examined under amicroscope, it is seen that the fingers 14 are joined to the arm 10 by awrist-like contraction 13 which is both exceedingly strong and has agreater warp force than the balance of the arm. FIG. 1 illustrates thenatural throwing arm, or seed-throwing extension of the seed pod of theplant Geranium dissectum, when the arm is in its dry condition. However,all of the plants of the genus Geranium have the same shape, differingfrom that of Geranium dissectum only in size and strength. FIG. 2 showsthe arm of FIG. 1 when it is in a fully saturated, and, therefore,straight condition. It perhaps should be mentioned here that whenstraight, these arms-are not limp-they straighten with the same forcethat they bend to the spiral form when drying. Hence, these fibersassume the same shape on both adsorption and desorption. A wet xericelement is analagous to a splinter or straight spine.

The arms as they exist naturally, after the throwing of the seeds, canbe used for very satisfactory xeric elements. They do respond to changesin relative humidity more rapidly than known xeric elements, theircurves of adsorption and desorption are closer together than heretoforeknown, and they do have a relatively long life as compared to elementsheretofore known. However, since the covering 15 (FIG. 5) of the spine,or rib, of the arms deteriorates more rapidly than the rib, suchdeterioration does affect the accuracy and response of the device inwhich they are contained. Accordingly, we prefer to form the xericelements by retting in the following manner: gynoecium of a suitabletype are collected after they have fully ripened and thrown their seeds.Preferably, the spiral throwing arms are removed from the gynoecium andare soaked in a warm water bath, preferably one having a, temperature ofbetween and F. The pod extensions, or arms, are allowed to ret, or soak,in the water for a period of approximately 3 weeks to permit bacteria toattack the thin fibrous coating 15 and beards, or barbs, 16 thereon. Theinner rib has been found to be very resistant to bacterial action andwill not rot away, as will the surface coating 15. It has been foundthat the action can be speeded up by increasing the temperature, but itappears that more consistent results are secured by slower retting inwarm water.

After the seed pods, or specifically the throwing arms, have beenallowed to ret for a sufficient period of time, the coating 15, beards16, and pocket 11 are stripped from the rib, which is then in arelatively straight form because of its saturated condition. Strippingcan be readily accomplished in a simple manner, such as by pulling thearms through the fleshy part of the tightly closed fingers of one handof an operator. Usually, it is not desirable to use finger nails orother sharp surfaces to grasp the rib, as that may cut, and thereforeweaken the final element. It is possible to determine when the podextensions are ready for stripping by examining their color. When thepods are first placed in the water, they have a relatively light browncolor, whereas, after the bacterial action, which results from theprolonged retting, they have a relatively dark brown color. Removing thecovering from the rib of the extension of the seed pod exposes the innerfiber rib which has a relatively smooth hard-looking appearance, such asshown in FIG. 4. If this rib is allowed to dry, it assumes the formshown in FIG. 3 which, for all practical purposes, is identical with theshape of the natural dried throwing arms shown in FIG. 1. A crosssectionof the throwing arm of Geranium dissectum is shown in FIG. it beingshown with its natural covering as in FIGS. 1 and 2. It has a flatdorsal side formed of tightly packed, longitudinal fibers 17 which haveelasticity in both wet and dry states but do not warp. The balance ofthe spine is formed of cellulosic material 18 with longitudinal fibers19, which warp as moisture conditions change. The warping phenomenon isassociated with the physics of cellulose crystallite structure and thenature of fibrillar structure in the xeric cells. It will be understoodby those skilled in the art that the retted fiber can be used as a xericelement in the form in which it appears in FIGS. 3 and 4. For example,if one end of the fibrous arm 10 were cemented to a fixed axis, and alight pointer were cemented to the free end of the element, a xerometerof satisfactory characteristics would be provided, as is shown in FIGS.5 and 6 of the copending application of Fenner, one of the inventorsherein, entitled Xerometer, Ser. No. 834,093, filed June 17, 1969, nowUS. Pat. No. 3,608,377. The trouble in using the element in this way,and without further preparation, is that the dial of registration wouldhave to have a dial extending through an angle of about 420 to perhaps540. Accordingly, it is preferable to use the partially bridled element,which will now be described.

It has been found that, for most practical applications of the xericelement of the present invention to measuring and other devices, it isbetter to bridle the movement of the xeric element as by securing theends of sections of the fiber to a bridling member, or saddle, such asone formed of a thin sheet of beryllium copper 25 (FIGS. 6 to 8,inclusive). Tests show that for practical purposes the bridled element,or chip, of satisfactory characteristics, can be obtained by usingasheet of beryllium copper which has a thickness of one onethousandthsof an inch, a width of 2 millimeters, and a length of 14 millimeters.The ends 26 of this saddle 25 are bent backwardly along a common face ofthe saddle for a distance of 2 millimeters to form the pockets 28, shownparticularly in FIG. 6. This saddle now accordingly has an overalldimension of 2 millimeters in width by l centimeter in length. The xericelements of Geranium dissectum have a warp force of about 5 grams each,though other species exert forces greater than 100 grams. In order toprovide a bridled element having movement through an angle ofapproximately 60, and in order to equalize small variations in thestrength of individual fibers, it is better to insert a plurality offibers (usually three) having a length of about 9 to 10 millimeters intothe bridling element 25. Since the wrist portion of the fiber is thestrongest portion and has a stronger warp force, we prefer to use the 9millimeter segment which includes the wrist 13 of the fiber at one end.These sections of xeric fiber are in a thoroughly saturated andtherefore an exceedingly straight condition, and are inserted by meansof tweezers into the pockets 28 formed between the major portion of thebridling element 25 and its reversed ends 26, with the dorsal side 17against the saddle 25. Each end of the combination is then wrapped withfine wire and immediately dipped in melted solder while still wet,whereby the solder 27 solidifies within the pockets 28, therebyanchoring the fiber elements 10 at their ends to the saddle. This isshown particularly well in FIG. 7, in which the fiber elements 10 are intheir fully saturated condition and the saddle, or bridling element 25retains its naturally straight configuration. If this chip, or bridledelement is permitted to dry, it has an arcuate shape in which the twopocketed ends form an angle of about 60 relative to each other, as shownin FIG. 8. The pockets permit movement which compensates for thedifferential coefficient of expansion between the xeric element and themetal.

Since the saddle and the fibrous element are united only at their ends,the bridling does not affect the natural warping actions of the fiber,but it does hold the fibers in the desired parallel position in whichall elements work together. If the element is not abused, it has anindefinite life and will retain its warp force response indefinitely.The selectivity of this xeric element to moisture, its capacity tomaintain equilibrium with moisture in almost any surrounding environmentwith a minimum hysteresis effect, its response at high and lowtemperatures, its ability to function under conditions of vacuum or upto 3,000 pounds per square inch environments, its strength, itsresistance to contamination, its small size, its capacity'to respondreliably at extremes of fully wet and fully dry conditions, are factorswhich make this element extremely useful. In addition, the device withwhich this xeric element can be associated, can be much simpler thanthose heretofore used in xeric measurements.

It will be understood by those skilled in the art that the plants of thegenus Geranium and especially the species mentioned in the claims, areillustrative of a larger group of seed-throwing plants. Hence, theinvention should not be deemed to be limited to this genus Geranium orto the particular species claimed, but to include any plant in which anextension of the seed pod, or husk, which, when dry, assumes asubstantially uni-planar spiral curve. It is known that cellulosegenerally has warping characteristics, but not all such materials areusable in xeric instruments for various reasons. For purposes ofinstrumentation, it is essential that the xeric fiber have a reversibleaction, so that it moves through the same curve on both adsorption anddesorption; that the response to changes in relative humidity be rapid;that the response to changes in relative humidity be constant over longperiods and under all conditions (i.e., that it be accurate); and thatit have sufficient inherent strength in both wet and dry conditions tonot break when bridled. In addition, it is highly desirable that itscurvature be substantially uni-planar, that its movement be in equalincrements directly proportional to each change in moisture content overa long range; and that it have substantially identical hysteresis curveson adsorption and desorption. Other desirable characteristics are thatit have a long life; that it have a low thermal coefficient so that itwill be affected by moisture only and not by temperature; that it havegood viscoelastic qualities so that it has elasticity even when bent toits limit of movement; and that it be of a shape and size to fit easilyinto small instruments. In all of these particulars, the plants of thegenus Geranium, and particularly the species herein mentioned,specifically fulfill all of these requirements, but other plants whichthrow" their seeds will have some of them and, accordingly, many of themcould be used for inferior instrumentation.

Likewise, many modifications in the form of the saddie, or bridling,element can be suggested by those skilled in the art. Obviously, thebridled element may contain one or a considerable plurality of thevegetable xeric elements; the saddle can be longer so as to usesubstantially all, or at least a larger portion of the fiber, orsmaller, to be incorporated directly into solid state circuitry; thefiber can be attached directly to piezoelectric strain gauges, such asthe Endevco Pixie, without the saddle; the fiber can be attached to thesaddle by other means, such as an epoxy glue, or the element could besecured to the saddle throughout its entire length, such as by cementingto a flat sheet of beryllium copper by an epoxy glue, and the like.Accordingly, it is intended that the appended claims cover suchmodifications as fall within the true spirit and concept of theinvention.

We claim:

1. A xeric element comprising the rib of the seed throwing arm of theseed pod of plants of the genus Geranium, said seed throwing arm in itsnatural state having said interior rib and an outer covering thereon,said outer covering being stripped away from said rib to expose the ribto the atmosphere by retting said arm and then stripping said coveringfrom said arm.

2. The element of claim 1 in which the plant is Geranium dissectum.

3. The element of claim 1 in which the plant is Geranium Richardsonii.

4. The element of claim Geranium attenuilobum.

5. The element of claim Geranium maculatum.

6. The element of claim Geranium eriostemon.

l in which the plant is I in which the plantis l in which the plant is7. A xeric element of claim 1 wherein the throwing arm of the seed podis taken from the class consisting of Geranium attenuilobum, Geraniummaculatum, Geranium eriostemon, Geranium Richardsonii, and Geraniumdissectum.

8. A xeric element comprising a saddle formed of a thin resilient metal,a moisture-sensitive fiber taken from the seed pod of a plant of thegenus Geranium, and means for securing the ends of the fiber to themetallic saddle with the length of the fiber lying along the metalsaddle.

9. A xeric element of claim 8 wherein the moisturesensitivefiber istaken from the spine of the seedthrowing arm of the seed pod of thegroup of plants consisting of Geranium dissectum, Geranium Richardsonii,Geranium attenuilobum, Geranium maculatum,

and Geran' m eriostemon.

10. The c ievice of claim 8 in which a plurality'of the lic elementcomprises a sheet of beryllium copper with a thickness of approximatelyone one-thousandths of an inch.

12. The method of preparing a xeric element which comprises forming asaddle out of a thin sheet of resilient metal, bending over the ends ofsaid sheet to form pockets at the two ends thereof, placing on the metalsheet and extending into each pocket thereof a plurality of the fibrousribs secured from retting the throwing arms of the seed pods of a plantof the genus Geranium, said arms being characterized by a soft fibrouscoating enclosing a relatively firm rib, said retting being for asufficient time to allow bacteria to attack the soft fibrous coating ofthe arm and then stripping the soft material from the relatively firmrib within the arm, and soldering each end of the metal sheet to securethe ends of the firm hard ribs within the pockets formed in the ends ofthe metal element.

13. The method of claim 12 wherein said plant is taken from the groupconsisting of Geranium dissectum, Geranium Richardsonii, Geraniumattenuilobum, Geranium maculatum and Geranium eriostemon.

14. A xeric element comprising the spine only of the seed-throwing armof the seed pod of plants of the genus Geranium, comprising also ametallic saddling element and means for securing the xeric elementthereto with the length of the fiber lying along the metal saddle.

1. A xeric element comprising the rib of the seed throwing arm of the seed pod of plants of the genus Geranium, said seed throwing arm in its natural state having said interior rib and an outer covering thereon, said outer covering being stripped away from said rib to expose the rib to the atmosphere by retting said arm and then stripping said covering from said arm.
 2. The element of claim 1 in which the plant is Geranium dissectum.
 3. The element of claim 1 in which the plant is Geranium Richardsonii.
 4. The element of claim 1 in which the plant is Geranium attenuilobum.
 5. The element of claim 1 in which the plant is Geranium maculatum.
 6. The element of claim 1 in which the plant is Geranium eriostemon.
 7. A xeric element of claim 1 wherein the throwing arm of the seed pod is taken from the class consisting of Geranium attenuilobum, Geranium maculatum, Geranium eriostemon, Geranium Richardsonii, and Geranium dissectum.
 8. A xeric element comprising a saddle formed of a thin resilient metal, a moisture-sensitive fiber taken from the seed pod of a plant of the genus Geranium, and means for securing the ends of the fiber to the metallic saddle with the length of the fiber lying along the metal saddle.
 9. A xeric element of claim 8 wherein the moisture-sensitive fiber is taken from the spine of the seed-throwing arm of the seed pod of the group of plants consisting of Geranium dissectum, Geranium Richardsonii, Geranium attenuilobum, Geranium maculatum, and Geranium eriostemon.
 10. The device of claim 8 in which a plurality of the moisture-sensitive fibers are secured to the metal saddle.
 11. The xeric element of claim 11 in which the metallic element comprises a sheet of beryllium copper with a thickness of approximately one onE-thousandths of an inch.
 12. The method of preparing a xeric element which comprises forming a saddle out of a thin sheet of resilient metal, bending over the ends of said sheet to form pockets at the two ends thereof, placing on the metal sheet and extending into each pocket thereof a plurality of the fibrous ribs secured from retting the throwing arms of the seed pods of a plant of the genus Geranium, said arms being characterized by a soft fibrous coating enclosing a relatively firm rib, said retting being for a sufficient time to allow bacteria to attack the soft fibrous coating of the arm and then stripping the soft material from the relatively firm rib within the arm, and soldering each end of the metal sheet to secure the ends of the firm hard ribs within the pockets formed in the ends of the metal element.
 13. The method of claim 12 wherein said plant is taken from the group consisting of Geranium dissectum, Geranium Richardsonii, Geranium attenuilobum, Geranium maculatum and Geranium eriostemon.
 14. A xeric element comprising the spine only of the seed-throwing arm of the seed pod of plants of the genus Geranium, comprising also a metallic saddling element and means for securing the xeric element thereto with the length of the fiber lying along the metal saddle. 